1. Field of the Invention
This invention relates to a molded core for an electric motor including a resin cover formed on an outer face of the core by means of injection molding.
2. Description of the Prior Art
FIG. 8 illustrates a conventional molded core. The molded core 10 comprises a stator core 1 and a resin cover 4 covering an outer face of the stator core. The stator core 1 is formed by stamping out a predetermined shape of steel sheet and stacking a number of such steel sheets. The stator core 1 includes an annular yoke 2 and a plurality of teeth 3 extending radially from an outer circumference of the yoke. In forming the resin cover 4, the stator core 1 is placed in a molding die (not shown). A molten insulating resin is injected into the molding die by an injection molding machine so that the resin cover 4 is formed by means of injection molding. The resin cover 4 usually has a substantially uniform thickness in its entirety.
In order that the molten resin may fill a cavity of the molding die at a uniform fluidity so that a distal end of each tooth 3 is immersed in it, a number of gates 5 through which the molten resin is injected into the die cavity are formed in the molding die so that distances between the gates 5 and the respective corresponding teeth 3 are equal to one another. However, the resin cover 4 has a relatively smaller and generally uniform thickness. Moreover, since the distance that the molten resin flows is increased as it approaches the distal end of the tooth 3, the fluid resistance is increased and accordingly the fluidity of the molten resin is reduced.
For the purpose of overcoming this problem, a maximum injection pressure is required when the molten resin is injected to fill the die cavity so that the end of each tooth 3 is immersed in it as shown in FIG. 9. More specifically, FIG. 9 shows variations in the injection pressure with progress of time in the process of injecting the molten resin. The molten resin is primarily injected to fill the die cavity so that the yoke 2 is immersed in it in a first half stage Q1, whereas the molten resin is primarily injected to fill the die cavity so that the ends of the teeth 3 are immersed in it in the second half stage Q2. As obvious from FIG. 9, the fluid resistance is small in the stage Q1 since the yoke 2 is located near the gates 5. As a result, the molten resin is injected with a small injection pressure in the stage Q1. On the other hand, the fluid resistance is large in the stage Q2 since the ends of the teeth 3 are located further away from the gates 5. Accordingly, the molten resin is injected with the maximum injection pressure X in the stage Q2.
The gates 5 are disposed so that the distances between the gates and the respective corresponding teeth 3 are equal to one another, as described above. Accordingly, the molten resin is injected so that the ends of the respective teeth 3 are synchronously immersed in it. As a result, the pressure is dispersed to the teeth 3 even when the molten resin is injected with the maximum injection pressure X. This reduces the substantial injection pressure for each tooth 3, whereupon there is a possibility that the molten resin may not reach the distal end of each tooth 3.
On the other hand, the prior art has proposed use of a large-sized injection molding machine having a high maximum injection pressure. However, such a large-sized injection molding machine is expensive, increasing the manufacturing cost of the motor. Further, the increase in the maximum injection pressure distorts the stator core 1 during the molding and/or results in a large number of fins protruding from a contact surface of the molding dies.
Therefore, an object of the present invention is provide a molded core which includes a resin cover formed on an outer face thereof by means of molding and in which as the result of an improvement in the fluidity balance of the molten resin, the molten resin can reliably be injected to fill the die cavity so that the distal end of each tooth is immersed in it.
To achieve the object, the present invention provides a molded core for an electric motor, which includes a resin cover formed by placing a stator core in a cavity of a molding die and injecting a molten insulating resin into the cavity of the molding die so that a resin cover is formed on an outer surface of the stator core by means of molding. The molded core comprises a generally annular yoke, a plurality of teeth radially extending from the yoke, the yoke and the teeth constituting the stator core, the teeth including alternately arranged first and second teeth, each first tooth having a larger radial dimension than each second tooth, and a plurality of distal end resin cover portions included in the resin cover and covering distal end faces of the teeth respectively, the distal end resin cover portions having different radial dimensions, the distal end resin cover portion of each second tooth having a larger radial dimension than the distal end resin cover portion of each first tooth.
According to the above-described construction, the fluidity of the insulating resin is improved in the distal end of each tooth having the distal end resin cover portion with the larger radial dimension. Accordingly, the insulating resin injected into the cavity of the molding die first flows to an outer face of each tooth having the distal end resin cover portion with the larger radial dimension. Thereafter,the insulating resin flows to an outer face of each tooth having the distal end resin cover portion with the smaller radial dimension. Thus, differing from the conventional construction, the above-described construction can increase the injection pressure. Consequently, the insulating resin can reliably be injected far into the distal end of each tooth.
Furthermore, the teeth include alternately arranged first and second teeth, each first tooth having a larger radial dimension than each second tooth, and the distal end resin cover portion of each second tooth has a larger radial dimension than the distal end resin cover portion of each first tooth. In this construction, the distance that the insulating resin flows is shorter in each second tooth than in each first tooth. Moreover, since the fluidity of the insulating resin is good in the distal end of each tooth, the resin can smoothly be injected to the outer face of each second tooth. Accordingly, the time the resin is injected so that the second teeth are immersed in it is retarded or quickened relative to the time the resin is injected so that the first teeth are immersed in it. Consequently, the injection pressure at which the resin is injected so that each tooth is immersed in it can be increased.
In another preferred form, a sum of the radial dimension of each tooth and the radial dimension of the distal end resin cover portion of each tooth is substantially uniform. When the molded core of the above-described construction is incorporated with a rotor, a mechanical air gap defined between the molded core and the rotor can be rendered substantially uniform over the entire circumference. Consequently, wind noise due to rotation of the rotor can be reduced, and the center of rotation of the rotor can easily be aligned with an axial center of the molded core.
In further another preferred form, the molding die has a plurality of gates through which the insulating resin is injected into the cavity of the molding die, the gates being formed in a portion of the molding die corresponding to the yoke and disposed near the teeth having the distal end resin cover portions with a larger radial dimension respectively. In this construction, the insulating resin can be injected to cover each tooth having the distal end resin cover portion more quickly than to the other teeth. Consequently, the time the molten resin is injected to cover each one tooth is retarded or quickened relative to the time the resin is injected to cover another tooth.
In further another preferred form, the molding die includes a first support pressed against a distal end face of each wide tooth of the stator core placed in the molding die so as to support the stator core and a second support pressed against a distal end face of each narrow tooth so as to support the stator core, and wherein each first support has a portion which is pressed against the wide tooth and which has an area larger than an area of a portion of each second support pressed against the distal end face of the narrow tooth. When the stator core comprises a stack of laminated steel sheets, the injection pressure inevitably displaces the laminated steel sheets radially. In the above-described construction, however, the radial displacement of the steel sheets can be reduced.